In a magnetic resonance device, such as a magnetic resonance tomography system, the body of a person to be examined (e.g., a patient) may be subjected to a relatively high magnetic field (e.g., 1.5, 3 or 7 Tesla) with the aid of a main magnet. Additionally, gradient pulses are applied with the aid of a gradient coil unit. Using a radio-frequency antenna unit, high-frequency radio frequency pulses, such as excitation pulses, are then sent out using suitable antenna devices. The high-frequency radio frequency pulses lead to the nuclear spin of specific atoms excited resonantly by the radio frequency pulses being flipped by a defined flip angle in relation to the magnetic field lines of the main magnetic field. During the relaxation of the nuclear spin, radio-frequency signals (e.g., magnetic resonance signals) are emitted. The radio-frequency signals are received by suitable radio frequency antennas and processed. The desired image data may be reconstructed from the raw data acquired.
For a specific measurement, a specific magnetic resonance sequence (e.g., a pulse sequence) including a sequence of radio-frequency pulses (e.g., excitation pulses and refocusing pulses) and gradient pulses is to be sent out. The radio-frequency pulses and gradient pulses are coordinated appropriately to be sent out in different gradient axes along different spatial directions. Read-out windows offset to match the pulses, which prespecify the periods of time in which the induced magnetic resonance signals will be detected, are set.
In magnetic resonance imaging using a magnetic resonance device, the homogeneity of a main magnetic field in an examination volume is of great importance. Even small deviations of the homogeneity may result in large deviations in a frequency distribution of the nuclear spin, so that lower-quality magnetic resonance image data is recorded.
In order to improve the homogeneity in the examination volume, shim units are used. When a magnetic resonance device is installed at its intended location, magnetic fields present in the environment may restrict the homogeneity of the main magnetic field, especially around an isocenter of the magnetic resonance device. Therefore, on installation and commissioning of a magnetic resonance device, frequently in conjunction with measurements, the shim unit is set so that homogeneity is optimized. Basic shim settings are computed during installation and commissioning of the magnetic resonance device.
A further source of inhomogeneity is represented, however, by the object to be examined. For example, if a person to be examined is introduced into the magnetic resonance device, the material of the body disrupts the homogeneity of the magnetic field. To counteract this problem, adjustable shim units are used. For example, electric shim coils that create different compensation magnetic fields when activated with different shim currents in order to improve the homogeneity are known.
In order to shim these disruptions of the object to be examined, upon activation of the shim unit, basic shim settings obtained during the installation and commissioning of the magnetic resonance device are used to take a measurement of the field distribution when the person to be examined is introduced into a patient accommodation area of the magnetic resonance device. Thereafter, starting from the basic shim settings, shim settings are established using the control unit, taking into account the measured field distribution. The shim unit is then activated using the shim settings in order to obtain an optimum possible homogeneity.